Minor changes to io/espresso

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.

These changes will add some flexibility and make it easier for other packages to import espresso IO functions. It should not affect ase.espresso.io in any way (at least that I can think of). I don't think this requires new tests?